From a viewpoint of improving yield in fabrication of semiconductor devices, management of surface conditions of a peripheral portion of a substrate has been attracting attention in recent years. In the fabrication process of the semiconductor devices, various materials are deposited on a silicon wafer to form a multilayer structure. As a result, unwanted films and roughened surface are formed on a peripheral portion of the substrate. It has been a recent trend to transport the substrate by holding only its peripheral portion using arms. Under such circumstances, the unwanted films remaining on the peripheral portion would be peeled off during various processes and could adhere to devices, causing lowered yield. Thus, in order to remove the unwanted films formed on the peripheral portion of the substrate, the peripheral portion of the substrate is polished using a polishing apparatus. In this specification, the peripheral portion of the substrate is defined as a region including a bevel portion which is the outermost portion of the substrate, and a top edge portion and a bottom edge portion located radially inwardly of the bevel portion.
FIG. 23A and FIG. 23B are enlarged cross-sectional views each showing a peripheral portion of a wafer as an example of a substrate. More specifically, FIG. 23A is a cross-sectional view of a so-called straight-type wafer, and FIG. 23B is a cross-sectional view of a so-called round-type wafer. In the wafer W shown in FIG. 23A, the bevel portion is an outermost circumferential surface of the wafer W (indicated by a letter B) that is constituted by an upper slope portion (an upper bevel portion) P, a lower slope portion (a lower bevel portion) Q, and a side portion (an apex) R. In the wafer W shown in FIG. 23B, the bevel portion is a portion (indicated by a letter B) having a curved cross section and forming an outermost circumferential surface of the wafer W. A top edge portion is a flat portion E1 located radially inwardly of the bevel portion B. A bottom edge portion is a flat portion E2 located on the opposite side of the top edge portion and located radially inwardly of the bevel portion B. The top edge portion may also include a region where the devices are formed.
As an apparatus for removing such films formed on the peripheral portion of the wafer W, there has been known a polishing apparatus using a polishing tool such as a polishing tape (for example, see Japanese Patent No. 5254575). This type of polishing apparatus has a substrate holder for holding a wafer W and rotating the wafer W, and a polishing head for bringing the polishing tape (polishing tool) into contact with the peripheral portion of the wafer W. The polishing head has a pressing pad for pressing the polishing tape against the peripheral portion of the wafer W. The pressing pad disposed on the reverse side of the polishing tape presses a polishing surface of the polishing tape against the peripheral portion of the wafer W, thereby polishing the peripheral portion of the wafer W. As the polishing tool, a strip-shaped polishing cloth may be used in place of the polishing tape.
FIG. 24 is a perspective view showing an example of a conventional pressing pad. As shown in FIG. 24, a pressing pad 150 has an elastic member 155 having a rectangular pressing surface 155a, and a pad body 154 to which the elastic member 155 is secured. The elastic member 155 is fixed to the pad body 154 in a state where an entire rear surface on an opposite side of the pressing surface 155a is brought in contact with the pad body 154. The pressing pad 150 is disposed on the reverse side of the polishing tape and presses the front surface (polishing surface) of the polishing tape against the bevel portion B of the wafer W by the pressing surface 155a of the elastic member 155. The elastic member 155 of the pressing pad 150 is made of a material such as rubber or sponge. For example, urethane rubber or silicon sponge with a hardness (e.g., 20 to 40 degrees) suitable for polishing a substrate is selected as a material of the elastic member 155.
FIG. 25 is a schematic view showing the state where the bevel portion B of the wafer W is polished by a polishing head 130 having the pressing pad 150 shown in FIG. 24. As shown in FIG. 25, the polishing head 130 has the pressing pad 150 for pressing a polishing tape 123 against the peripheral portion of the wafer W, an air cylinder (driving mechanism) 152 for moving the pressing pad 150 toward the peripheral portion of the wafer W, and a tape feed mechanism 142 for feeding the polishing tape 123 in a predetermined direction. The force of the pressing pad 150 that presses the polishing tape 123 against the wafer W is regulated by controlling air pressure supplied to the air cylinder 152. During polishing of the bevel portion B of the wafer W, the polishing head 130 (i.e., the pressing pad 150) is inclined with respect to the wafer W by a tilting mechanism (not shown). While the polishing head 130 is inclined with respect to the wafer W, the pressing surface 155a of the elastic member 155 of the pressing pad 150 presses the polishing tape 123 against the bevel portion B of the wafer W, thereby polishing the entire bevel portion B by the polishing tape 123.
When the polishing head 130 (i.e., the pressing pad 150) is inclined with respect to the wafer W to polish the entire bevel portion B, the width of the polishing tape 123 which is brought into contact with the bevel portion B of the wafer W is changed. FIG. 26 is a schematic view showing the width of the pressing surface 155a of the elastic member 155 which is brought into contact with the bevel portion B of the wafer W through the polishing tape 123 when the pressing surface 155a of the elastic member 155 of the pressing pad 150 shown in FIG. 24 is perpendicular to a flat surface of the wafer W. FIG. 27 is a schematic view showing the width of the pressing surface 155a of the elastic member 155 which is brought into contact with the bevel portion B of the wafer W through the polishing tape 123 when the pressing surface 155a of the elastic member 155 of the pressing pad 150 shown in FIG. 24 is inclined with respect to the flat surface of the wafer W. In FIGS. 26 and 27, although the polishing tape 123 is not shown for the sake of simplifying the explanation, the width of the polishing tape 123 which is brought into contact with the bevel portion B of the wafer W during polishing corresponds to the width of the pressing surface 155a of the elastic member 155 which is brought into contact with the bevel portion B of the wafer W through the polishing tape 123.
As shown in FIGS. 26 and 27, the width Wa of the polishing tape which is brought into contact with the bevel portion B of the wafer W when the pressing surface 155a is perpendicular to the flat surface of the wafer W is smaller than the width Wb of the polishing tape which is brought into contact with the bevel portion B of the wafer W when the pressing surface 155a is inclined with respect to the flat surface of the wafer W. As the inclination angle of the pressing surface 155a with respect to the flat surface of the wafer W becomes larger, the width of the polishing tape which is brought into contact with the bevel portion B of the wafer W becomes larger.
FIG. 28 is a photograph showing polishing impressions formed in the polishing tape 123 when the bevel portion B is polished by pressing the polishing tape 123 against the bevel portion B of the wafer W with the conventional pressing pad 150. FIG. 28 shows a plurality of polishing impressions when the bevel portion B of the wafer W has been polished by changing the inclination angle θ of the pressing surface 155a with respect to the flat surface of the wafer W every 10 degrees. FIGS. 29A, 29B and 29C are schematic views showing the inclination angle θ of the pressing surface 155a with respect to the flat surface of the wafer W. In FIGS. 29A, 29B and 29C, the polishing tape 123 is shown as a vertical cross section along a center of the pressing surface 155a. As shown in FIG. 29A, this inclination angle θ is zero degree when the pressing surface 155a of the elastic membrane 155 of the pressing pad 150 is perpendicular to the flat surface of the wafer W. This inclination angle θ becomes a value of plus when the pressing surface 155a is inclined in a direction where the upper end of the pressing surface 155a approaches the flat surface of the wafer W as shown in FIG. 29B, and becomes a value of minus when the pressing surface 155a is inclined in a direction where the upper end of the pressing surface 155a moves away from the flat surface of the wafer W as shown in FIG. 29C.
As can be seen from the photograph shown in FIG. 28, as an absolute value of the inclination angle θ becomes larger, the polishing impression becomes larger in length. For example, the length La of the polishing impression when the inclination angle θ is 0° is smaller than the length Lb of the polishing impression when the inclination angle θ is 70°. The difference in length between the polishing impressions corresponds to the difference in width between the polishing tapes 123 which are brought into contact with the bevel portion B of the wafer W. As the width of the polishing tape 123 which is brought into contact with the bevel portion B of the wafer W is smaller, the polishing tape 123 which contributes to polishing of the bevel portion B is smaller in amount. As a result, the polishing rate when the absolute value of the inclination angle θ of the pressing surface 155a is small is lower than the polishing rate when the absolute value of the inclination angle θ of the pressing surface 155a is large.
Further, as can be seen from the photograph shown in FIG. 28, when the absolute value of the inclination angle θ is small, brightness of color at a central region of the polishing impression is higher than brightness of color at an outer region of the polishing impression. As shown in FIG. 30, the difference in brightness of color of the polishing impression means that a pressing force Fa at a central part of a contact area between the polishing tape 123 and the bevel portion B of the wafer W is larger than a pressing force Fb at an outer part of the contact area between the polishing tape 123 and the bevel portion B of the wafer W. In this case, the polishing tape 123 is liable to be clogged at the central part of the contact area of the polishing tape, thus lowering the polishing rate.